In high-speed on-the-fly line printers a plurality of print hammers are usually arranged in a row and are selectively operated to strike the type faces on a constantly moving type carrier such as a revolving flexible band, belt, chain or train or a rotating drum. The type carrier is moving very fast, and good print registration requires that the hammers be controlled so that each print hammer is actuated to cause the hammer impact to occur exactly when the desired type face is aligned with the selected print hammer/print position. Furthermore, it is desirable to terminate the actuation of the print hammer after a specified actuation period.
The time period or flight time between hammer actuation and impact is different for each print hammer and also may vary during extended use. Numerous control means have been employed for incorporating variable delay circuits into the hammer firing circuits, whereby the hammers are individually controlled so that impact occurs at the exact time that the desired type face becomes aligned with the selected print hammer location.